(POLITICO) — Inside a sleek Denver condominium, George W. Bush let a hundred donors to his brother’s campaign in on a secret. Of all the rival Republican candidates, there is one who gets under the former president’s skin, who he views as perhaps Jeb Bush’s most serious rival for the party’s nomination.

It isn’t Donald Trump, whose withering insults have sought to make Jeb pay a political price for his brother’s presidency. It also isn’t Marco Rubio, Jeb’s former understudy who now poses a serious threat to his establishment support.

It’s George W. Bush’s former employee — Ted Cruz.

“I just don’t like the guy,” Bush said Sunday night, according to conversations with more than half a dozen donors who attended the event.

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